A river on a diet
The Piave River flows 220 kilometers from its source in the Eastern Alps to the Adriatic Sea, shaping the Venetian plain. For millennia, this flow carried a heavy load of sand, silt, and gravel eroded from the mountains. This sediment was the primary building material for the coastline, creating and sustaining the sandy beaches and barrier islands that protect the historic city of Venice and its lagoon. Before large-scale human intervention, the river transported millions of cubic meters of this material to its delta annually, a natural process of coastal nourishment that balanced the erosive power of the sea.
This natural sediment supply has been drastically cut. The primary cause is the extensive network of hydroelectric dams built along the Piave and its tributaries, mostly between the early 20th century and the 1960s. The Piave basin is one of the most heavily engineered for hydropower in Europe. It contains a system of 13 major dams, 12 artificial lakes, and dozens of power plants that harness the river's energy. These structures generate about 6% of Italy's entire energy demand, they also trap sediment behind their walls. The result is a dramatic reduction in the amount of sand and gravel reaching the coast, a condition known as sediment starvation.
The hungry water effect
The consequences of trapping nearly all the river's solid load are twofold. First, the coastline is deprived of its source of replenishment. Without a steady supply of new sand, the natural process of coastal erosion caused by waves and currents begins to dominate, leading to retreating shorelines. The popular tourist resort of Lido di Jesolo, which depends on the Piave's sediment, has experienced severe erosion, necessitating the construction of extensive coastal defenses.
Second, the water released from the dams is "hungry." Stripped of its sediment load, the clear water has excess energy, which it expends by scouring the riverbed downstream. This process has caused the river channel to deepen by up to 3 meters in some places. The river, which once deposited material to build up the plain, now erodes its own bed. This sediment deficit drives the coastal retreat and subsidence affecting the entire northern Adriatic coast, compounding the threats posed by rising sea levels to the city of Venice.